Generally, in the communication system, an increase in the number of mobile stations increases the interference caused by multiple access, causing a reduction in the channel capacity. To prevent the reduction in the channel capacity, a receiver (or receiving terminal) uses an interference canceler. The interference canceler can be classified into a Parallel Interference Canceler (PIC) and a Successive Interference Canceler (SIC). The interference cancelers repeatedly perform data detection and interference cancellation to cancel pilot signals and data signals of other mobile stations, which serve as interference signals to the receiver. Implementation of the receiver for performing such operations is very difficult. In order to solve such problems, a pilot interference canceler has been proposed, which cancels only the pilot cancelable without the detection process, from the signal received at the receiver.
Because the pilot signal cannot be used for data transmission and reception, it acts as overhead in terms of the system capacity. As an example of the overhead, when the number of mobile stations connected to a base station increases, only some mobile stations transmit data, and the remaining mobile stations may transmit only the pilot signal and control signal necessary for link maintenance, without transmitting the data. In this case, therefore, an increase in the system capacity can be expected by applying the pilot interference canceler to the communication system.
FIG. 1 illustrates a structure of a receiver with a conventional pilot interference canceler.
Referring to FIG. 1, if the receiver disables the pilot interference canceler, a signal received via an antenna is input to a modem receiver (or CDMA modem receiver) 114 after passing through a Radio Frequency (RF) front-end unit 102. The modem receiver 114 performs modem processing on the input signal.
However, when the receiver uses the pilot interference canceler, the signal output from the RF front-end unit 102 is input not only to the modem receiver 114, but also to a sample buffer 104 and an addition and subtraction unit 112. Herein, the pilot interference canceler includes the sample buffer 104, a controller 106, L fingers 108-1 to 108-L, and addition and subtraction units 110 and 112.
The sample buffer 104 consecutively stores the signal output from the RF front-end unit 102, and delivers the stored signal to the fingers 108-1 to 108-L. The total number L of fingers is determined depending on the maximum number of simultaneously accessible mobile stations supportable by the pilot interference canceler and the number of fingers allocated per mobile station. The fingers 108-1 to 108-L each estimate and regenerate a pilot signal corresponding to a multi-path channel of each mobile station. For the regeneration of the pilot signal, the fingers 108-1 to 108-L should receive a Pseudo Noise (PN) code of the mobile station to which they are allocated, and a finger offset caused by a channel delay, provided from the modem receiver 114. To this end, the controller 106 serves to deliver the PN code and finger offset information to the fingers 108-1 to 108-L.
The pilot signals regenerated in the fingers 108-1 to 108-L are added up by the addition and subtraction unit 110, and the resulting added signal is cancelled from the received signal. As a result, the pilot interference canceler generates a pilot signal-cancelled received signal.
FIG. 2 illustrates a detailed internal structure of the finger.
Referring to FIG. 2, a signal output from the sample buffer 104 is input to an interpolator 202. The interpolator 202 interpolates the received signal taking the finger offset into account, and converts the interpolated signal into a chip-based signal.
A PN generator 206 generates a local PN code synchronized to the corresponding finger taking into account a long code mask and a finger offset. The signal output from the interpolator 202 and the PN code output from the PN generator 206 undergo despreading through multiplication by a multiplier 204, and then output to a rake processor 208.
The rake processor 208 accumulates the input signal at intervals of N chips to increase a Signal-to-Noise Ratio (SNR). Herein, N is an implementation parameter, which is determined depending on a chip rate, the maximum change rate of the channel, etc. The rake processor 208 separately accumulates the signal to be used as an input to the channel predictor 214 and the signal to be used as an input to the frequency offset estimator 210.
The channel predictor 214 calculates a channel gain using the pilot symbol, which has been accumulated in units of N chips. The frequency offset estimator 210 estimates a frequency error using the pilot symbol, which has been accumulated without frequency offset compensation.
A pilot regenerator 212 regenerates a pilot signal using the output signal of the PN generator 206, the channel gain y(m) estimated by the channel predictor 214, and the frequency offset {circumflex over (ε)}(m) estimated by the frequency offset estimator 210
A pilot SNR estimator 216 estimates an SNR of the pilot signal using the received signal from the sample buffer 104 and the channel gain y(m) from the channel predictor 214. A scaling factor determiner 218 calculates a scaling factor α(m) using the estimated SNR of the pilot signal. The scaling factor calculated in this way is multiplied by the signal output from the pilot regenerator 212, generating an output signal of a kth finger.
As described above, the pilot interference canceler regenerates and then adds up pilot signals of all mobile stations belonging to a particular cell or sector separately for each finger, and subtracts the result from the received signal. Performance of the pilot interference canceler is determined depending on how the estimation on the pilot signal included in the received signal is accurate. The pilot signal estimation accuracy of each finger is determined depending on the performance of the channel predictor, and the performance of the channel predictor is subject to change according to the SNR of the pilot signal. That is, when the SNR of the pilot signal is high, the accuracy of the channel predictor increases, decreasing the signal estimation error, and when the SNR of the pilot signal is low, the accuracy of the channel predictor decreases, increasing the signal estimation error.
The scaling factor calculated by the scaling factor determiner 218 of FIG. 2 serves to increase a cancellation rate of the pilot signal when the SNR of the pilot signal is high, and to decrease the cancellation rate of the pilot signal when the SNR of the pilot signal is low, thereby minimizing the amount of a residual interference signal included in the pilot signal-cancelled received signal. In order to calculate the scaling factor, there is a need for the noise power and pilot channel power estimated by the pilot SNR estimator. However, because the estimation on the pilot channel power is performed using the output signal of the channel predictor, an estimation error may occur during the scaling factor determination. In addition, when the estimation error of the channel predictor and the SNR of the finger have a nonlinear characteristic, the process of calculating a scaling factor from the estimated SNR is complicated, causing a dramatic increase in the implementation complexity.